TITLE:   GCN CIRCULAR
NUMBER:  29775
SUBJECT: GRB 210406A: Mondy optical afterglow candidate
DATE:    21/04/08 20:53:54 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI), N. Pankov (HSE), A. Pozanenko (IKI), E. Klunko (ISTP) 
report  on behalf of GRB IKI FuN:

We observed the field of GRB 210406A (Mereghetti  et al., GCN 29767) 
with AZT-33IK telescope of Sayan observatory (Mondy) in second epoch 
starting on 2021-04-07 (UT) 16:54:14. The first epoch was obtained early 
(Pankov  et al., GCN 29770). We obtained 20 images in R-filter. In 
comparison of the sources we found one fading source in coordinates of 
(J2000) 08:49:49.09    76:33:20.2 with accuracy of 0.5 arcsec in each 
coordinate. The source is faded by 0.5mag between the two epochs. 
Preliminary photometry of the source in  stacked images is following

Date       UT start   t-T0       Filter Exp.   OT    Err.  UL(3sigma)
                     (mid, days)        (s)

2021-04-06 17:54:58   0.04069    R     15*120  21.90  0.14  22.9
2021-04-07 16:54:14   0.98811    R     20*120  22.41  0.14  23.2

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id RA DEC R2
1665-0048950 132.414156 +76.555625 16.49
1664-0050141 132.551831 +76.468300 18.01

The source is not presented in Pan-STARRS PS1 but marginally  visible in 
Pan-STARRS (r-plate) and presented in Legacy Survey DR8.

We suggest the source as afterglow candidate of optical afterglow of GRB 
210406A.